Preheat oven to 375 degrees. Spray an 8" X 8" baking pan with nonstick spray.
Bring a large skillet sprayed with nonstick spray to medium-high heat. Cook and stir onion until slightly softened, about 5 minutes. Add tomatoes and cook until softened, about 2 minutes. Transfer to a large bowl, and blot away excess moisture.
Add spinach to the skillet and cook until just wilted, about 2 minutes. Transfer to a strainer, and press with paper towels to remove as much liquid as possible.
Transfer spinach to the large bowl. Let cool slightly, about 5 minutes.
Add all remaining ingredients to the bowl, and mix well. Transfer to the baking pan.
Bake until center is firm, about 1 hour. Serve and enjoy!